Unblinding and demand characteristics in the treatment of depression

Blinding of treatment allocation in clinical trials in psychiatry is regarded as an ideal. The potential impact of unblinding chimes with a general concern for psychological research: so-called demand characteristics can undermine confidence in findings from experimental and clinical studies. Scepticism can result in nihilism. The reliance on subjective report of symptoms in clinical trials of drug efficacy in depression provides an important example. It is regularly implied that if subjective effects, including specific adverse reactions, unblind participants to an active treatment then evidence for its efficacy is suspect. In fact, the strong association between dose and subjective effects does not translate into a strong relationship with efficacy in randomised controlled trials (RCTs) of conventional antidepressant drugs; this observation falsifies the proposition that unblinding is the principal mechanism driving RCT outcomes in studies of depression. Instead, changes in brain function, that occur soon after treatment starts, do predict treatment outcomes and align with our understanding of neurotransmitter effects from neuroscience. Psychedelic experience for the treatment of depression must be unblinding, but the effect results directly from serotonergic receptor activation and changes in brain connectivity. Where such effects are part of a novel mechanism of action, a strong dose response relationship would be expected, irrespective of unblinding. We highlight the importance of exploring blinding as a mechanism, confirming dose-related outcomes, and dissociating unblinding effects from efficacy. Unblinding does not necessarily invalidate the subjective experience of sustained recovery from depression.